To be suitable for use as magnetic field sensors such as Hall sensors and magnetoresistors, it is well known that semiconductors and semiconductor films must have a sufficiently high electron mobility to provide good sensitivity to a magnetic field. It is also well known that such semiconductor devices must simultaneously have a sufficiently high electron density to obtain good temperature stability, so that sensor output is in response to changes in the immediate magnetic field and not in response to changes in temperature.
Electron density can be improved by homogeneously doping the semiconductor material with properly selected elements to increase the number of free electrons in the semiconductor material. These doping impurities serve as either donors or acceptors to the semiconductor compound to provide free carriers, in the form of electrons or holes, respectively, by which current may be conducted through the semiconductor. However, increased dopant density also inherently results in increased scattering of the free carriers, and thereby reduces the sensitivity of the magnetic field sensing device by decreasing the scatter-free length of an electron's path through the semiconductor. Accordingly, attempting to improve electron density by doping the semiconductor material is often self-defeating in that electron mobility is effectively reduced due to scattering on dopant impurities. Thus, a compromise must often be made between optimizing the carrier mobility and the carrier density within the operating temperature range of the device.
An alternate approach to the above homogeneous-doping method is referred to as slab-doping, which entails growing a thin layer of doped semiconductor material which interfaces with an undoped semiconductor layer. The electrons or holes released from the impurities in the doped layer diffuse into the undoped layer of the semiconductor film where they will have higher mobilities than in the doped layer due to lower dopant density. The intent is that the mobility of the semiconductor film, averaged over the doped and the undoped layers, will be higher than that of a homogeneously doped semiconductor film with the same average impurity concentration. Using this approach, multiple layers of doped material can be alternated with layers of undoped, or intrinsic, material.
There are a number of parameters that can be adjusted to obtain the desired improved average mobility in a slab doped semiconductor film, including doped layer thickness, impurity concentration in the doped layer, the number of alternating doped layers, and the thickness of the intrinsic layer between doped layers. Where the resulting semiconductor film has a slab-doping profile that consists of multiple alternating layers of heavily doped n-type material and intrinsic material, the structure is commonly referred to as a "nini"-structure. Where the thickness of each n-type doped layer in a nini-structure is limited to a single atom plane, the slab-doping process is referred to as ".delta.-doping."
A major difficulty with all slab-doping techniques is that the presence of the doped layer electrostatically distorts the conduction and valence band edges in the intrinsic material near the doped layer, such that electrons are attracted to the positively charged donor ions, thereby reducing the mobility of the electrons. This effect is due to the attractive Coulomb potential V which the ionized impurities impose upon the free electron carriers. The energy bands then bend by an amount qV, where q is the electron charge. The Coulomb potential is always attractive since the ionized impurity has an opposite charge to that of the free carrier, whether an electron or hole, which it has released. The bending of the bands always tends to form a Coulomb potential well around the impurity. In a randomly doped sample, the Coulomb potentials around each impurity tend to average out over the whole sample, leaving only a small ripple in the semiconductor band edges. This ripple affects transport properties only at very low temperatures below 1K and in very high magnetic fields of about 10 tesla or greater.
However, in a .delta.-doped sample there is a large density of charged impurities in one atomic plane. The Coulomb potential well is then very deep and can completely confine the electrons of the impurity, preventing it from serving as a donor or acceptor. Even if the Coulomb potential well does not confine every electron, it will confine a substantial fraction of the total electron density close to the impurity-containing layer, reducing the average carrier mobility by reducing the probability that the electrons diffuse into the undoped, or intrinsic, semiconductor layer.
Other negative influences on electron mobility include film defects (dislocations and boundaries) and spin-disorder scattering. Film defects include the boundary effects of the interface between the film and the substrate or air and dislocations which result mostly from lattice mismatch between the film and the substrate. Spin-disorder scattering of the free electrons occurs when a dopant atom exhibits a magnetic moment as a result of having an unpaired electron in its outer shell. As a result, the path of an electron through the semiconductor is altered when it passes near such a dopant atom, thus decreasing the overall carrier mobility of the semiconductor by making the path of the electron through the semiconductor irregular. As an example, rare-earth elements are known to exhibit such a magnetic moment, causing spin disorder scattering of electrons which reduces the carrier mobility in the semiconductor.
In the prior art, the rare-earth elements europium and ytterbium, are known to have been added to narrow-gap (i.e. a narrow band gap between the valence and conduction bands on the order of 0.1 to 0.3 eV) Group IV-VI semiconductors to increase the energy band gap of the semiconductor material. A number of other rare-earth elements such as dysprosium, holmium, erbium and gadolinium are known to have been added to such narrow-gap Group IV-VI semiconductors to dope these semiconductors to n-type conductivity. Also, rare-earth elements have been added to large-gap III-V compounds (i.e. a band gap between the valence and conduction bands on the order of 1.4 eV) to create additional energy levels in the band gap of the semiconductor material when used as a dopant. Both of these uses of rare-earth dopants are directed toward opto-electronic devices and not magnetic field sensing devices.
Doping a semiconductor with rare-earth atoms can be advantageous in that the magnetic effects of a single atom can be studied even in relatively concentrated alloys. The 4f wavefunctions are extremely local in nature such that there is virtually no overlap with the 4f wavefunctions of the nearest neighbor. However, rare-earth atoms can reside on different sites of the host crystal lattice, which complicates the interpretation of the observed effects. Furthermore, there could be interactions between rare-earth atoms affected by the conduction electrons of the semiconductor. Accordingly, the effect of rare-earth atoms on a semiconductor material is often unpredictable.
With reference again to electron mobility and density in a semiconductor film, the above discussion is particularly applicable to indium antimonide (InSb), a narrow-gap Group III-V compound which is often the preferred semiconductor compound for magnetic field sensor devices. This is because InSb has, among all binary compounds, the highest electron mobility at room temperature. InSb also has a very low energy band gap (approximately 0.18 eV) between its valence and conduction bands. Unfortunately, this energy band gap is sufficiently small to enable electrons to readily jump from the valence band to the conduction band when exposed to modest increases in temperature, making the electron density in the conduction band of InSb sensitive to temperatures near and above room temperature. Accordingly, InSb films must be doped to improve their temperature stability. However, improvements in temperature stability with previous attempts at doping InSb, such as nini-doping with silicon, have resulted in impaired electron mobility.
Thus, it would be desirable to provide a dopant which induces a high carrier density in an InSb semiconductor film to improve its temperature stability while also obtaining a high carrier mobility of the film so as to enable its use as a magnetic field sensing device.